1. Field of the Invention
The present invention relates to filters, and, more particularly, to a filter cartridge for a liquid-straining filter of the type which is cylindrical in shape and designed for a radial flow direction of the liquid.
2. Description of the Prior Art
From the prior art it is known to produce filter cartridges which consist of a central perforated sleeve around which are wound a number of overlying layers of creped filter paper, confined in the axial sense between two non-permeable end discs.
Filter assemblies with cartridges of this type are known as depth filters, the primary purpose of the coiled, layered structure being to utilize successive layers over the radial depth of the cartridge for the retention of solid deposits, in order to prevent the premature clogging of the peripheral entry surface of the filter cartridge. On the other hand, the filter material is to have fine pores distributed evenly over its surface, in order to remove as much as possible of the fine solid matter from the passing liquid.
Some known depth filters utilize fiber layers in their filter cartridges. These fiber layers, however, are frequently subject to a non-uniform distribution of the fiber material, in addition to the possibility of packing together in certain areas, with the result that the capacity of the filter cartridge is impaired accordingly. It has therefore become more common to utilize filter cartridges which feature a spiral coil of one or two strips of filter paper. The use of filter paper greatly facilitates the control of the pore size and pore distribution and, consequently, of the resistance of the filter paper to the liquid medium which is to be filtered.
One such depth filter is disclosed in the German Published Application C 41 47, where the filtering surface in an annular paper cartridge is increased by using one or several sheets of creped filter paper with alternating longitudinal edges connected together, the paper being wound around a central sleeve to form a spiral coil. However, this type of filter cartridge allows only axial flow-through, and it is therefore suitable only for filter assemblies which are designed for such an axial flow. Special precautions have to be taken, in order to prevent the mounting of the filter cartridge in an axially reversed orientation.
Another filter cartridge is disclosed in the German Gebrauchsmuster (Utility Model) No. 77 08 832. This solution proposes the use of a continuous web of paper fleece which is wound around a perforated central sleeve in the form of a spiral coil, to form a radially permeable body. The superposed layers of this coil have aligned longitudinal edges which abut against two metal end discs of the cartridge. This structure has the shortcoming of forcing the entire liquid flow through the outermost layer of the filter coil, so that it tends to develop premature clogging on its outer surface and its radial depth is not utilized effectively.